Cardiovascular disease, including atherosclerosis, is the leading cause of death in the United States. The medical community has developed a number of methods and devices for treating coronary heart disease, some of which are specifically designed to treat the complications resulting from atherosclerosis and other forms of coronary artery narrowing.
An important development for treating atherosclerosis and other forms of coronary narrowing is percutaneous transluminal coronary angioplasty, hereinafter referred to as "angioplasty" or "PTCA". The objective in angioplasty is to enlarge the lumen of the affected coronary artery by radial hydraulic expansion. The procedure is accomplished by inflating a balloon within the narrowed lumen of the coronary artery. Radial expansion of the coronary artery occurs in several different dimensions, and is related to the nature of the plaque. Soft, fatty plaque deposits are flattened by the balloon, while hardened deposits are cracked and split to enlarge the lumen. The wall of the artery itself is also stretched when the balloon is inflated.
Unfortunately, while the affected artery can be enlarged, in some instances the vessel restenoses chronically, or closes down acutely, negating the positive effect of the angioplasty procedure. In the past, such restenosis has frequently necessitated repeat PTCA or open heart surgery. While such restenosis does not occur in the majority of cases, it occurs frequently enough that such complications comprise a significant percentage of the overall failures of the PTCA procedure, for example, twenty-five to thirty-five percent of such failures.
To lessen the risk of restenosis, various devices have been proposed for mechanically keeping the affected vessel open after completion of the angioplasty procedure. Such endoprostheses (generally referred to as "stents"), are typically inserted into the vessel, positioned across the lesion or stenosis, and then expanded to keep the passageway clear. The stent overcomes the natural tendency of the vessel walls of some patients to restenose, thus maintaining the patency of the vessel.
Various types of stents are currently under development, although to date none has proven completely satisfactory during testing. U.S. Pat. No. 4,655,771 to Wallsten describes a stent comprising a tube of stainless wire braid. During insertion, the tube is positioned along a delivery device, such as a catheter, in extended form, making the tube diameter as small as possible. When the stent is positioned across the lesion, it is expanded, causing the length of the tube to contract and the diameter to expand. Depending on the materials used in construction of the stent, the tube maintains the new shape either through mechanical force or otherwise.
U.S. Pat. No. 4,733,665 to Palmaz describes a stent comprising a slotted stainless steel cylinder that forms a mesh when expanded. The stent is delivered to an affected area by a balloon catheter, and is then expanded to the proper size by inflating the balloon.
Stents are typically delivered to affected areas of vessels using standard catheterization techniques. A thin walled hollow guiding catheter is introduced into the body via a relatively large vessel, such as the femoral artery in the groin area or the brachial artery in the arm by insertion through a hollow sheath. The guiding catheter is maneuvered through an approximately 180 degree turn through the aortic arch to descend into the aortic cusp where entry may be gained to either the left or the right coronary artery, as desired.
A flexible guidewire is inserted into the guiding catheter and advanced to the area to be treated. The guidewire is advanced across the lesion in preparation for the advancement of a catheter across the guide wire. Typically a balloon catheter carrying the stent is then introduced over the guidewire to the area to be treated. Radiopaque markers in the balloon segment of the catheter facilitate positioning across the lesion. The balloon catheter is then inflated with contrast material to permit fluoroscopic viewing during treatment. The balloon is inflated until the lumen of the artery is satisfactorily enlarged and the stent is in place.
During delivery of the stent, the stent and balloon catheter must navigate narrow tortuous vessels into the site of a stenosis. Typically, the balloon and stent are covered with a sheath during delivery. Such sheaths facilitate delivery of stents because the sheath prevents the stent from being dislodged from the delivery device. However, sheaths increase the cross-sectional profile of the stent, necessitating use of a guiding catheter with a larger internal diameter. The large diameter of the guiding catheter may increase the risk of complications at the patient access site. Moreover, the larger cross-sectional profile of the stent delivery system may decrease the ability to deliver contrast material through the guiding catheter to enable precise positioning. More important, an increased cross-sectional profile may make it impossible to deliver a stent through narrow and tortuous vessels to the area desired to be treated.
Stent delivery systems without sheaths have been proposed. For example, the Boneau stent described in U.S. Pat. No. 5,292,331 has been used with a modified delivery device described in co-pending U.S. patent application Ser. No. 08/451,270 which is hereby incorporated by reference. The disclosed delivery system secures the stent on the outside of the balloon without the need for a sheath.
However, attempts to deliver expandable stents without using a delivery sheath have resulted in a number of problems. First the relatively rigid stent may be dislodged from the more pliable balloon material of the flexible delivery device when contact occurs between the vessel wall and the stent, particularly during passage through a curve. Additionally, the low mass of the rigid stents, and construction methods of some stents, causes the end portions to have small cross-sections and to be somewhat ridged and sharp. The small, sharp, cross-sections at the ends of the stents increase the risk that a stent will penetrate the vessel wall, particularly when the narrow, rigid end of a stent encounters a curve in a vessel. The flexible delivery device and balloon material follow the contours of the vessel, and when negotiating a curve, the distal end of the more rigid stent may separate slightly from the balloon and delivery device. The separated, sharp distal end may cause abrasion or penetrate the soft tissues of the vessel wall.
These and other complications have resulted in a low level of acceptance for such stents within the medical community for certain procedures, and to date stents have not been accepted as a practical method for treating many chronic restenosis conditions.
It would therefore be desirable to provide methods and apparatus, useful for treating chronic restenosis conditions, that enable delivery of a stent without a sheath and which reduce the risk of vessel abrasion and dissection during delivery.